Ceramic materials are typically prepared from powder precursors and heat treated or sintered such that the individual grains solidify into a solid body. In many cases, the desired composition includes small amounts of a second composition added to enhance the sintering process, such as to decrease porosity or lower the temperature at which the body may be sintered. Small chemical additions, known as dopants, are used to enhance sintering and adjust material properties. These additions are typically added in small amounts, less than 10% and more commonly in the one to two percent level. The distribution of these dopants is typically uncontrolled or ‘left to chance’, insofar as it is done in such a way as to result in a non-uniform distribution of the dopants throughout the compact. Such a non-uniform distribution leads to defects in the resultant compact.
The current approach for introducing dopants falls into two general categories: salt solutions and the use of second phase particles. Salt solutions are initially perfectly mixed, as the salt concentration within the solution should be ideally uniform. As water evaporates from the particle compact, or even within a droplet during a spray-drying process, the salt migrates with the water in the packed particle structure. The last pocket of liquid, be it in the meniscus between two particles or a reservoir within an agglomerate, is where the salt will concentrate. Once the solubility limit is reached, the salt precipitates. If more than one dopant is used, then the precipitation process occurs sequentially, with the lowest solubility salt precipitating first, etc. This can, and does, typically lead to large-scale segregation of the dopants.
In systems which use second phase particles, the dopant particles are often of similar size as the primary material particles. In some cases, the dopants are often several orders of magnitude larger, leading to gross segregation as the system is being milled or mixed. Large dopant particles inherently lead to large-scale segregation and, typically, to having significant excess dopant in the system. Because of large-scale segregation, additional dopant is used to compensate for segregation. This is often leads to inferior or variable material properties after sintering. Thus, there remains a need for a process that more uniformly distributes sintering dopants throughout the sintered body.
Another area of ceramic processing suffering form inhomogeneous distribution of components is wash-coats. Current technology for wash-coats consists of a mixture of ceramic particles, including typically two or more of the following: alumina, silica (quartz and cristobalite), zirconia, and colloidal silica (amorphous silica). All of these particles have different surface chemistries in water, meaning that the surface charges can be opposite on different particles and of different magnitudes. This leads to severe problems in maintaining suspension stability over the lifetime of the slurry pots. When the suspension becomes unstable, often indicated by excessive settling or gelation of the suspension, the pot is discarded, often meaning that several tons of material are simply dumped. Having uncontrolled surface chemistries means that the pH of the suspension shifts and therefore must be adjusted on a daily basis.
One source of loss for investment casting systems is the failure of the shell during the cast, resulting in ceramic shell fragments in the molten metal, resulting in the rejection of the cast and significant financial loss. Slurry processing of the powders to create the composite grains ensures excellent mixing of the component powders, resulting in a significant improvement in the strength of the ceramic shell, thus potentially reduces losses associated with shell failure.
Thus, there is a need for a powder processing technique for producing two-phase ceramic powder systems that do not suffer from uneven distribution of phases and material (both chemical and physical) properties. The present novel technology addresses these needs.